The goal of this project is to improve the outcome of bone marrow transplantation for leukemia by delivering supplemental radiation directly to bone marrow, spleen, and lymph nodes using monoclonal antibodies to which radioactive iodine-131 (131I) is attached. Upon being injected into the blood, these antibodies circulate and bind to their target proteins on leukemic and normal cells in these organs. The additional dose of radiation to these sites where leukemia cells reside may decrease the risk of relapse after bone marrow transplant without increasing toxicity of the procedure, because the delivery of "targeted" radiation relatively spares normal tissues such as the liver, lung, and kidney. In this pilot study we are using growth factor to increase the number of cells in the bone marrow in an attempt to test whether this increase in the number of potential binding sites actually improves the targeting of radiolabeled antibody to the bone marrow. Our initial experience demonstrated that the targeting of radiolabeled antibody was more effective in patients whose marrows were very full of leukemia cells than in those whose leukemia was in remission, i.e., whose marrow was only 50% full of cells. Pilot studies of two growth factors suggested that one of these factors, G-CSF, could produce the desired doubling of cells in the marrow; this effect persisted for several days after the growth factor was stopped, thus allowing a window during which the white blood cell count in the blood had fallen but the marrow was still quite cellular. Now we are planning to test whether increasing the number of cells in the marrow with the growth factor actually results in the predicted increase in the localization of the radiolabeled antibody to marrow.